#include <stdio.h>

int main(int argc, char const *argv[])
{
    int arr[] = {1,2,3,4,1,3,4};
    int len = sizeof(arr) / sizeof(int);
    int m = 0;

    for(int i = 0; i < len; i++)
        m ^= arr[i];

    printf("%d\n",m);

    return 0;
}
